1. Field of the Invention
The present invention relates to a reversibly changeable heat-sensitive recording medium which has a reversibly changeable heat-sensitive layer and heat-dependent transparency, and, based on temperature changes, can allow repeated recording and erasing. In further detail, the present invention relates to the aforementioned reversibly changeable heat-sensitive recording medium which has excellent visibility.
2. Description of the Prior Art
Previously, as heat-sensitive recording media for recording or erasing reports on plastic, there were media having a heat-sensitive layer which comprises resin matrices such as polyvinyl chloride, polyester, polyamide, etc., containing higher alcohols, low molecular weight higher fatty acids dispersed in the matrix. Images are formed or erased through the use of the reversibly changeable temperature dependent transparency changes from transparent to opaque or from opaque to, transparent.
In this type of heat-sensitive recording medium, there is a large difference between the light-scattering strength of the opaque and transparent portions and the visibility of the recorded portions is good. Attempts have made to increase the difference between the light-scattering strength of the opaque and transparent portions by changing the matrix, low molecular weight organic material composition or the thickness of the heat-sensitive layer, by increasing the light-scattering strength of the opaque portion, by controlling the conditions of the low molecular weight compound dispersed in the matrix, and by lowering the light-scattering strength of the background of the transparent portion, as a process for obtaining good visibility. (See Japanese Patent A-Publication Nos. 54-119377 (1979), 55-154198 (1980), and 64-14077 (1989).) However, in these processes the materials, heat-recording equipment, and manufacturing method were all fixed and the improvement in the visibility was limited.
In recent years, a process to improve the visibility has been proposed whereby a fluorescent dye is added to the heat-sensitive layer and the protection layer prepared on top of the heat-sensitive layer, etc. (See Japanese Patent A-Publication Nos. 3-253388 (1991), 7-101186 (1995).)
However, when using materials which absorb UV radiation in the protective layer prepared on the top of the heat-sensitive layer, the efficacy of the fluorescent body which has an excitation wavelength in the ultraviolet region deteriorates. Furthermore, when the background is made of the material which has light scattering strength of the same order as the opaque portions, the difference between the light-scattering strength of the opaque portions and the transparent portions is reduced and there is a disadvantage in that the visibility decreases.